New applications continue to be developed for radio signaling in the microwave and higher frequency ranges. For example, certain scanning radar systems operating in the range of 77 GigaHertz (GHz) can provide collision warning and avoidance information for controlling motor vehicle traffic. In such a system, moving and stationary obstacles in front of the vehicle are detected by the radar system. Post-processing modules analyze the radar data and, when necessary, the driver is alerted. In critical situations (when driver reaction is too slow), such systems can also be used to automatically apply the brakes. Other developed technologies in this area relate to adaptive cruise control of vehicle systems, which adapt the speed and distance of a vehicle to a preceding vehicle. The required functionality and reliability of such systems can typically be met through a combination of Monolithic Microwave Integrated Circuit (MMIC) based radar front-end electronics, and advanced antenna and signal processing for horizontal and vertical resolution, and microprocessor-implemented modules for evaluation of risk of collision, and strategies for informing the driver and braking the vehicle.
Other emerging applications for microwave signalling include the implementation of wireless data transmission systems. Such systems hold the promise of reduced network build out costs, especially in areas where telephone cable and high speed fiber optic lines are not available. Indeed, certain radio bands have already been dedicated to provide so-called Local Multipoint Distribution Service (LMDS) using high frequency microwave signals in the 28 or 40 GHz band. In the typical LMDS system, a hub transceiver services several different subscriber locations located within a given area, or cell, approximately up to six miles in diameter.
The implementers of vehicle radar, data transmission, and other microwave radio systems continue to be faced with several challenges at the present time. One challenge is in the electronics technology needed to implement these systems. Transceiver components must provide precise control over signal levels in order to effect the maximum possible link margin at the receiver. In addition, these systems must typically use a highly directional (i.e., narrowly focused) antenna that has very low cross polarization levels. The transceiver equipment, including the antenna, also typically needs to be small, compact, and light weight.
These requirements have led to the use of antennas for both LMDS service and microwave radars that use a so-called folding optics design. Such a design uses a device known as a transreflector placed in a plane orthogonal to the intended axis of the antenna and a twist reflector assembly also placed in the same plane. This type of antenna typically requires fabrication of multiple individual components. See, for example, the antennas described in U.S. Pat. No. 5,455,589 issued to Huguenin, G. R. and Moore, E. L. on Oct. 3, 1995 and assigned to Millitech Corporation, the assignee of the present application, as well as U.S. Pat. No. 5,680,139 issued on Oct. 21, 1997 to the same inventors, also assigned to Millitech Corporation.
Generally, the transreflectors used in these designs are fabricated as a structure with a curved surface on which a grid of fine parallel wires is disposed at closely spaced intervals. The interval spacing depends upon the frequency of the radio energy expected to be transmitted or received by the antenna. The grid serves as a polarizer for electromagnetic radiation, and the convex surface functions as a focusing reflector for the component of radiation having a polarization parallel to the wires.
Various techniques have been employed to manufacture such transreflectors. These techniques have generally involved a tedious and difficult alignment of wires along a closely spaced grid or other techniques for removing metal to leave a grid of finely spaced conductors. However, it is essential to the optimum operation of the transreflector that the conductive strips be absolutely parallel and uniformly spaced at small intervals. Precision alignment and spacing is often difficult to obtain with such procedures and achieving the required degree of precision economically is quite difficult. It is also desirable that such antennas be manufactured from low cost materials, using low cost processes as much as possible.